Related Applications
There are no applications for patent relating hereto heretofore filed in this or any foreign country.
1. Field of Invention
Our invention relates generally to solid state track recorders to measure radiation spectra and particularly to such recorders that measure time orientation of radiation.
2. Description of Prior Art
The nuclear arts, especially in their application to reactor environments, have required the development of neutron dosimeters to provide sensitive and accurate means of quantifying high fluence neutron radiation. In years past, neutron dosimetry has been carried out with radiometric dosimeters (RM), helium accumulation fluence monitors (HAFM), and, especially with the advent of automated scanning systems, with solid state track recorders (SSTR). The SSTR neutron dosimeters have become comparable in cost with RM and HAFM devices and have advantages over either of the later devices in neutron dosimetry.
SSTR dosimeters require less sensor material than RM dosimeters. Sensor material of the RM devices must be of high purity and usually involves use of U-238 or Np-237, both which are of limited supply and high cost. In fact RM devices could deplete the world supply of their sensor materials in commercial application if such dosimeters were exclusively used.
SSTR dosimeters are capable of higher efficiency then other dosimeters and are in fact sensitive enough to permit surveillance dosimetry on the exterior of a nuclear pressure vessel in a reactor cavity formed between the pressure vessel and its requisite biological shield. SSTR's also have lower radioactivity characteristics than the RM devices to provide reduction of personnel radiation exposure as well as simplified shipping and handling. Of particular advantage in this detection system is the microscopic spatial resolution afforded by SSTR neutron dosimetry.
All of the noted neutron dosimeters have been utilized for the surveillance of nuclear pressure vessels utilized in the nuclear power industry to ascertain neutron activity therein or thereabout. This surveillance is desirable in determining the deleterious effect of neutron particles on a pressure vessel, as metalurgical embrittlement will occur after exposure for a period of time and accordingly such risks need to be evaluated in light of the information thusly gathered. Neutron induced pressure vessel embrittlement has been recognized as a serious problem for many years and need has steadily grown to determine neutron activity about reactor pressure vessels to aid in evaluating the safety and useful life of a vessel. Such data is also useful in evaluating and possibly modifying equipment, systems, and procedures to accommodate measured neutron activity.
Our systems possess advantages for power reactors cavities, the cavity being the annular region between the pressure vessel and the surrounding biological shield. Use of our system in a reactor cavity would determine the time dependence of pressure vessel neutron exposure on a daily or finer basis. This time dependence can be significent especially for low leakage cores that are used by nuclear power utilities. At the same time, our system utilizes SSTR fission deposits that are easy to fabricate and quantify, since these deposits are almost 10,000 times more massive than the ultra low level SSTR deposits normally produced for surveillance dosimetry in power reactor cavities. In fact, recent efforts make ultra low level actinide SSTR deposits for cavity surveillance dosimetry reveal serious unresolved problems in background contamination and non-uniformity.
The advent of present day burst type fusion reactors has presented a new set of problems in this area. The fusion reactor creates intense radiation bursts, at the present time of necessarily limited time duration. Normal passive detection systems are not easily applied to the measurement of these high intensity radiation bursts in the limited time available. Our detection system has been developed particularly to overcome deficiencies of the past in monitoring and measuring the time dependence of neutron intensity in such burst type radiation fields.
We provide two relatively rotatable elements, the first defining axially spaced openings which may optionally support a series of threshold fission sources to be activated by incoming neutron flux to be measured. The second element views the fission sources and supports appropriate recording material to record evidence of neutron interactions therein. The point where fission fragment tracks being on the recording material determines the start of a neutron burst to be recorded, and if the relative rotational speed of the elements is appropriate, the time interval of a burst's duration can be tracked to its end on the recording media. This reaction orientates the entire track record in time and provides means by which time-dependent neutron intensity and absolution neutron fluence spectra may be deduced by appropriate calculation.
Prior art devices generally have measured the neutron flux, but have not recorded the time-dependence as found, for example, in burst fusion reactors. Our invention differs from the prior art in this regard, though its differences are not limited only to this feature, but rather resides in the synergistic combination of all of its elements that provide the functions necessarily flowing therefrom.